Electrostatic precipitators have been in use for many years and are very effective in removing very fine particulate from the atmosphere which otherwise would go unfiltered by merely mechanical means. Removing grease from the cooking exhaust atmosphere requires a specialized electrostatic precipitator, particularly when the grease is both electrically conductive and can arrive at the electrostatic precipitator in rather large droplets. This onerous operating condition can even be made worse by the addition of a high water content in the atmosphere, which commonly occurs when a new batch of frozen french fries is deposited into a deep fryer in a typical fast-food restaurant. Water boils off the frozen french fries almost immediately, thereby creating a very high humidity atmosphere in addition to the grease particulate.
Conventional electrostatic precipitators that remove particulate from cooking exhaust gases have been constructed of one or more cell assemblies-each containing a set of collecting plates and an ionizer-that are fed high-voltage electrical power from an electrical feed plenum located behind the cell assembly compartment(s). Each cell assembly essentially had an independent power feed. In addition, the high-voltage feed plenum required that the overall cabinet have a much greater depth than that required for the cell assemblies alone. Other conventional such electrostatic precipitators have been constructed by feeding the high-voltage electrical power from the side of the cell assemblies, however, the collecting cells of each cell assembly compartment were accessible only from the sides of the electrostatic precipitator so that any interior cells (rather than the outermost cells) had to be slid along a longitudinal rack for insertion or removal.
One primary concern with the use of electrostatic precipitators in removing electrically conductive grease particulate is the fact that the grease tends to accumulate in undesirable places, such as on electrical insulators, and this grease must be periodically removed. In view of this fact, such electrostatic precipitators must be placed into a "cleaning" mode after a certain well defined number of operating hours, depending upon the design of the electrostatic precipitator. A major limitation of conventional electrostatic precipitators is that this cleaning mode must occur quite frequently to keep the equipment in good operating order, and to prevent a dangerous situation in which the grease can catch on fire if a sufficient concentration occurs at a location having a high voltage gradient.
Grease fires are a common occurrence for this type of electrostatic precipitator equipment, as evidenced by the fact that such units are normally required to contain some type of fire or smoke detector system, and a fire extinguishing system. The problem of this electrically conductive grease collecting on the collecting elements and other components of the electrostatic precipitator is so insidious that some conventional designs actually construct some type of pan or trough to hold the liquid grease as it accumulates on the collecting components and drains downward toward the bottom of the precipitator, so that the grease can accumulate in a location that may not have electricity passing over it. In some designs, such a trough or pan can actually have holes to automatically drain the grease from the electrostatic precipitator. The major problem with a system having drain holes is that, since the grease atmosphere is typically arriving from directly below the electrostatic precipitator (i.e., the electrostatic precipitator is mounted directly above the deep fryers, as in a conventional hood arrangement), any type of hole in the bottom surface of the electrostatic precipitator will tend to allow a certain amount of "blow-by" of particulate that will not be channeled into the main collecting elements of the electrostatic precipitator. Any such blow-by will reduce the efficiency of the electrostatic precipitator, mainly due to the fact that a certain percentage of the particulate are never filtered at all. Some conventional electrostatic precipitators introduce clean air at their inlet to dilute the contaminated air and help clean the insulators; this arrangement obviously reduces the efficiency and the cleaning capacity of such convention electrostatic precipitators.
In an electrostatic precipitator designed to collect electrically-conductive grease, it is beneficial to provide as much electrical insulation as possible at all of the high-voltage elements within the system, and it is beneficial to direct any potentially accumulating grease away from such insulative elements. It is also beneficial to prevent as much as possible any blow-by of particulate around the main collecting elements of the electrostatic precipitator, and to construct the system in which the time intervals between mandatory cleanings is as long as possible. It is additionally beneficial to provide as much safety as possible during normal operation and cleaning operations of the electrostatic precipitator.